High density integrated semiconductor random access memory (RAM) is reaching the gigabit scale, led by the progressive evolutionary development of dynamic RAM (DRAM). The 1T/1C DRAM cell, consisting of a pass transistor and a capacitor, has the smallest possible ceil size, ranging between 4F.sup.2 and 8F.sup.2, where F is the minimum feature size. However, DRAM requires periodic refreshing, on the order of once per millisecond, since a bit is stored as charge on a capacitor, and the charge leaks away, at a rate of approximately 1 fA (10.sup.-15 A) per cell.
Static RAM (SRAM) provides enhanced functional qualities; no refreshing is needed and it is also generally faster than DRAM (approximately 25 nsec for SRAM compared to approximately 80 nsec for DRAM). However, the SRAM cell is more complex, requiring either six transistors or four trnsistors and two polysilicon load resistors, resulting in a cell size approximating 60 F.sup.2. It would be highly desirable to have memory cells with the functional qualities of SRAM, but with cell density closer to that of DRAM.
A resonant tunneling diode (RTD) in its simplest form consists of a sequence of five semiconductor layers. The outer two layers are the contact layers into which electrons enter and exit the semiconductor layer sequence. The interior three dissimilar semiconductor layers differ in their energy band gaps in the sequence wide/narrow/wide band gap with layer thicknesses comparable to the electron Bloch wavelength (typically less than 10 nm). This sequence of layers produces an energy profile through which electrons must travel and which consists of two energy barriers separated by a narrow region referred to as a quantum well.
Classically, an electron with energy, called the Fermi energy, approaching the first energy barrier with an energy below the barrier energy is reflected, analogous to a baseball rebounding off a concrete wall or to an electromagnetic wave at the end of an open-circuited transmission line. Quantum mechanics, however, allows that as the physical dimensions of the barrier decrease toward the wavelength of the particle, there is an increasing probability that the particle will be transmitted instead of reflected. Thus under certain conditions an electron can pass through the barrier even with energy below the barrier potential. This classically-forbidden phenomenon is called tunneling.
If the quantum well width is selected to be approximately equal to some integer or half-integer multiple of the electron wavelength, a standing wave can be built up by constructive interference analogous to the standing waves in a microwave cavity. Electrons at these wavelengths couple into and out of the quantum well more readily than others.
The electron energy, E, and its wavelength, 1, are inversely related by the equation, E=h.sup.2 /2 ml.sup.2, where h is Planck's constant and m is the effective electron mass. Since the electron's energy can be controlled by adjusting the bias across the structure, the transmission (or current flow) through the double-barrier depends sensitively on the applied voltage. One can think of the double-barrier structure as an energy bandpass filter which transmits for certain applied biases and reflects the electron for other applied biases. The electron is said to be in resonance when the incoming electron energy matches the resonant transmission energy of the quantum-well structure.
In the RTD, the current increases monotonically with applied voltage until the average incoming electron energy is approximately equal to the resonance energy and the electron tunnels efficiently through the double-barrier structure. At slightly higher energy (applied bias) the electron no longer couples into the well efficiently and the transmission (current) is reduced. At still higher applied voltages, the electron's energy is sufficient for it to get over the barriers giving rise to an increasing current with bias. Thus the current-voltage characteristic of the resonant tunneling diode is N-shaped. It is this characteristic which is utilized to advantage in resonant tunneling electron devices.
RTDs are often used in logic and analog signal processing circuits. They display a multistate and/or multilevel switching characteristic that is very useful in reducing the size, power dissipation or delay of conventional circuits. However, the operation of an RTD often creates a problem due to its inherent electrical hysteresis. Once switched from a low to a high voltage or current state, a reset of its applied bias is required to return the device to its original state.
The traditional Goto cell, which is disclosed in E. Goto, IRE Trans. Electronic Computers, March 1960, at p. 25, consists of a pass transistor 10 and two RTDs 12 and 14, and is shown in FIG. 1a, has the advantage of compactness and being static. FIG. 1b illustrates a load line analysis of the circuit of FIG. 1a, showing the two stable latching points 16 and 18. However, for RTDs with sufficient current drive, the valley current causes large static power dissipation. Thus, the idea of a "gain stage" to amplify the action of the RTD latch pair arises.
For CMOS gigabit DRAM, a number of 2-transistor (2T) gain cells have been proposed, four of which are shown in FIGS. 2a through 2d, to reduce the required storage node capacitance. The circuit of FIG. 2a is disclosed in H. Shichijo et al., Ext. Abs. 16th Int. Conf. on Solid State Dev. and Mat. (1984), p. 265. The circuit of FIG. 2b is disclosed in W. Kim, IEEE J. Solid State Circuits, vol. 29 (1994) at p. 978. The circuit of FIG. 2c is disclosed in S. Shukuri, Int. Electron Dev. Meeting Tech. Digest (1992) at p. 32.8.1. The circuit of FIG. 2d is disclosed in M. Terauchi, 1993 Symp. VLSI Tech. Digest, at p. 21. Advantages of these cells include high noise margin, ultra-low static power, and high current drive capacity. Disadvantages of these circuits are that the refresh requirement remains necessary, and multi-state operation is not possible.